Patent application title: Charging method

Abstract:

A charging method includes first and second charging steps to charge a
lithium-ion battery. In the first charging step, a temperature rise
gradient of the battery to a current is detected. A battery temperature
when the battery is charged to a first predetermined capacity is
predicted based on the gradient. A charging current is controlled based
on the predicted temperature. The battery is charged, at a current that
brings a battery temperature lower than a predetermined temperature, to
the first predetermined capacity. In the second charging step, a
temperature rise gradient of the battery is detected. A battery
temperature when the battery is charged to a second predetermined
capacity is predicted based on the gradient. A charging current is
controlled based on the predicted temperature. The battery is charged, at
a current that brings the temperature of the battery lower than the
predetermined temperature, to the second predetermined capacity.

Claims:

1. A constant-voltage and constant-current lithium-ion rechargeable
battery charging method comprising:a first charging step that detects, at
a current that is currently applied to a battery to be charged, a
temperature rise gradient of the battery, predicts a battery temperature
when the battery will be charged to a first predetermined capacity based
on the detected temperature rise gradient, controls a charging current
based on the predicted temperature, and charges the battery at a current
that brings a temperature of the battery lower than a predetermined
temperature to the first predetermined capacity; anda second charging
step that, after the battery is charged to the first predetermined
capacity, detects, at a current that is currently applied to the battery
to be charged, a temperature rise gradient of the battery, predicts a
battery temperature when the battery will be charged to a second
predetermined capacity based on the temperature rise gradient, controls a
charging current based on the predicted temperature, and charges the
battery at a current that brings a temperature of the battery lower than
the predetermined temperature to the second predetermined capacity.

2. The charging method according to claim 1, wherein first, second and
third predetermined temperatures T1, T2 and T3 that decrease in this
order are stored, and a temperature zone in that a charging current of
the battery is controlled is divided into first, second, third and fourth
temperature ranges, wherein the first temperature range is a range higher
than the first predetermined temperature T1, the second temperature range
is a range between the first and second predetermined temperatures T1 and
T2, the third temperature range is a range between the second and third
predetermined temperatures T2 and T3, and the fourth temperature is a
range lower than the third predetermined temperature T3, wherein if the
prediction temperature of the battery to be charged falls within the
first temperature range, a charging current is set to the minimum
current, if the prediction temperature of the battery to be charged falls
within the second temperature range, a charging current is reduced a
predetermined amount, if the prediction temperature of the battery to be
charged falls within the third temperature range, the battery is charged
without changing a charging current, and if the prediction temperature of
the battery to be charged falls within the fourth temperature range, a
charging current is increased a predetermined amount.

3. The charging method according to claim 1, wherein the predetermined
temperature is stored, wherein if a battery temperature is higher than
the predetermined temperature, a current for charging the battery is
reduced, and if a battery temperature is lower than the predetermined
temperature, a current for charging the battery is increased.

4. The charging method according to claim 3, wherein as the temperature
difference between the predetermined temperature and the prediction
temperature increases, a charging current is controlled by changing the
amount of current to be reduced or increased.

5. The charging method according to claim 1, wherein the first
predetermined capacity is a capacity in a range of 60% to 90% of the
fully-charged capacity.

6. The charging method according to claim 1, wherein the second
predetermined capacity is the fully-charged capacity.

7. The charging method according to claim 1, wherein the predetermined
temperature is a temperature in a range of 40.degree. to 45.degree. C.

8. The charging method according to claim 1, wherein in the first charging
step, a charging current for charging the battery is increased based on
the detected temperature rise gradient.

9. The charging method according to claim 1, wherein in the first charging
step, the battery is charged at a constant current, and in the second
charging step, the battery is charged at a constant voltage.

10. The charging method according to claim 1, wherein a voltage of the
battery is detected, and a charging current is controlled based on the
detected voltage, wherein if a voltage of the battery increases to a
predetermined voltage, a charging current for charging the battery is
reduced.

11. The charging method according to claim 10, wherein in the step in that
a voltage of the battery is detected, a charging current is controlled
based on the detected voltage, in that, if a voltage of the battery
increases to a predetermined voltage, a charging current for charging the
battery is reduced, and in that, after a voltage of the battery increases
to the predetermined voltage, a charging current is reduced, detection of
a voltage of the battery is stopped in a predetermined period after it is
detected that a battery voltage increases to the predetermined voltage.

12. The charging method according to claim 10, wherein in the step in that
a voltage of the battery is detected, a charging current is controlled
based on the detected voltage, in that, if a voltage of the battery
increases to a predetermined voltage, a charging current for charging the
battery is reduced, the battery temporarily stops being charged at the
timing where a charging current is reduced.

13. The charging method according to claim 1, wherein a voltage of the
battery is detected, and a charging current is controlled based on the
detected voltage, wherein if a voltage of the battery increases to a
predetermined voltage value, a charging current is reduced, wherein the
battery is charged at smaller one of a charging current that is
controlled based on the detected voltage, and a charging current that is
controlled based on the prediction temperature.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to a lithium-ion rechargeable battery
charging method, and in particular to a charging method that detects a
temperature of a battery and controls a charging current based on the
detected temperature. In this specification, a "lithium-ion rechargeable
battery" is used in a sense that includes a lithium-polymer battery.

[0003]2. Description of the Related Art

[0004]In order to improve safety, a charging current and a charging
voltage of lithium-ion rechargeable batteries are controlled according to
a temperature of the lithium-ion rechargeable batteries. FIG. 1 shows an
exemplary relationship of a charging current and a maximum voltage to a
temperature of lithium-ion rechargeable battery. As shown in this Figure,
a voltage and a current of lithium-ion rechargeable battery are set high
in a normal temperature range, but both charging voltage and current of
lithium-ion rechargeable battery are set low in high and low temperature
ranges. For this reason, if a temperature of a battery rises to the high
temperature range in charging operation, a charging voltage is reduced,
and as a result the battery cannot be fully charged. In conventional
constant-current and constant-voltage charging operation of a lithium ion
battery, in particular, at charging operation start, in the case where a
temperature of the battery is high, for example, as shown in FIG. 1, if a
battery temperature rises from the normal temperature range to the high
temperature range, charging voltage and current in the high temperature
range are lower than in the normal temperature range. For this reason,
there are problems in that the completed charged amount of the battery is
low, or charging operation time is long. In the case of a charging
circuit that cannot reduce a charging voltage, there is a problem in that
the charging circuit stops charging operation and as a result the
completed charged amount of the battery is low.

[0005]A method has been developed which prevents that a temperature of a
charged battery rises too high. This method is disclosed in Japanese
Laid-Open Patent Publication No. H07-73907-A (1995). In this method, a
temperature of nickel-hydrogen battery is held in a range of 10°
C. to 40° C. in charging operation. In addition to this, in this
charging method, a battery temperature rise value ΔT is measured,
and a battery temperature at charging operation completion is predicted
based on a battery temperature T and a value ΔT/Δt. If it is
determined that the predicted battery temperature will exceed 40°
C., the battery is cooled.

[0006]Also, a method has been developed which detects a temperature of a
battery and controls a charging current. This method is disclosed in
Japanese Laid-Open Patent Publication No. H07-222374-A (1995). This
method also charges a nickel-hydrogen battery. A temperature rise
gradient is detected, and a temperature is predicted based on the
detected temperature rise gradient. Accordingly, a charging current is
controlled based on the predicted temperature.

SUMMARY OF THE INVENTION

[0007]The charging method in Japanese Laid-Open Patent Publication No.
H07-73907-A (1995) requires a mechanism that forcedly cools the battery.
For this reason, there is a disadvantage in that manufacturing cost is
high. Also, since the battery is cooled by forcedly blowing air, there is
a disadvantage in that the noise level of a fan or the like that cools
the battery increases. Additionally, since mechanical parts such as the
fan are likely to develop trouble as compared with electronic circuits,
there is a disadvantage in that trouble is likely to occur. Also, in the
charging method in Japanese Laid-Open Patent Publication No. H07-222374-A
(1995), although a temperature of the charged battery can be low by
controlling a charging current based on a temperature rise gradient, in
the case where lithium-ion rechargeable batteries are charged in this
method, there is a disadvantage in that time to fully charge the
batteries remarkably increases.

[0008]The present invention has been developed for solving the
aforementioned disadvantages. It is an important object of the present
invention is to provide a charging method that can prevent temperature
rise of a lithium-ion rechargeable battery even in the state where the
battery is not forcedly cooled, and can fully charge the battery in a
short time.

[0009]A charging method according to the present invention is a
constant-voltage and constant-current lithium-ion rechargeable battery
charging method that includes first and second charging steps, and
charges a lithium-ion rechargeable battery by using the first and second
charging steps. In the first charging step, at a current that is
currently applied to a battery to be charged, a temperature rise gradient
of the battery is detected. A battery temperature when the battery will
be charged to a first predetermined capacity is predicted based on the
detected temperature rise gradient. A charging current is controlled
based on the predicted temperature. The battery is charged, at a current
that brings a temperature of the battery lower than a predetermined
temperature, to the first predetermined capacity. In the second charging
step, after the battery is charged to the first predetermined capacity, a
temperature rise gradient of the battery is detected at a current that is
currently applied to the battery to be charged. A battery temperature
when the battery will be charged to a second predetermined capacity is
predicted based on the temperature rise gradient. A charging current is
controlled based on the predicted temperature. The battery is charged, at
a current that brings a temperature of the battery lower than the
predetermined temperature, to the second predetermined capacity.

[0010]The aforementioned charging method has an advantage in that
temperature rise of a lithium-ion rechargeable battery can be prevented
without forcedly cooling the battery, and the battery can be fully
charged in a short time. The reason is that, according to the
aforementioned charging method, a battery is charged in the first
charging step, in that, at a current that is applied to a battery to be
charged, a temperature rise gradient of a battery at a charging current
is detected, in that a battery temperature when the battery will be
charged to a first predetermined capacity is predicted based on the
predicted temperature rise gradient, in that a charging current is
controlled based on the predicted temperature, and in that the battery is
charged, at a current that brings a temperature of the battery lower than
a predetermined temperature, to the first predetermined capacity, and
then, the battery is charged in the second charging step in that a
temperature rise gradient of a battery at a charging current is detected
at the current that is currently applied to a battery to be charged, in
that a battery temperature when the battery will be charged to a second
predetermined capacity is predicted based on the temperature rise
gradient, in that a charging current is controlled based on the predicted
temperature, and in that the battery is charged at a current that brings
a temperature of the battery lower than a predetermined temperature. In
particular, the charging method according to the present invention has a
feature in that a battery is charged to the first predetermined capacity
in the first charging step, and is then charged the second predetermined
capacity in the second charging step. Since, according to the present
invention, a battery is charged in a plurality of steps in that
predetermined capacities are different, the battery can be fully charged
in a short time.

[0011]FIG. 2 shows the state where a lithium-ion rechargeable battery is
charged in the aforementioned charging method. In this Figure, a curve
line A shows a charging current, and a curve line B shows the variation
of a temperature of the battery in this case. It can be understood from
this Figure that, according to the present invention, since a battery is
charged so that a charging current is controlled based on a prediction
temperature that is predicted based on a temperature rise gradient in the
first and second charging steps, the battery is charged at a current that
is high but brings a prediction temperature lower than a predetermined
temperature, and as a result the battery can be charged to the first
predetermined capacity in a short time. According to the present
invention, therefore, time to fully charge a battery can be short. In
addition to this, in both the first and second charging steps, since a
charging current is controlled to bring a temperature of the battery
lower than the predetermined temperature, a temperature of the battery
does not rise higher than the predetermined temperature.

[0012]In the aforementioned charging method, the first predetermined
capacity can be a capacity in a range of 60% to 90% of the fully-charged
capacity. Also, in the aforementioned charging method, the second
predetermined capacity can be the fully-charged capacity, and the
predetermined temperature can be a temperature in a range of 40°
to 45° C.

[0013]Also, in the aforementioned charging method, in the first charge
step, a charging current for charging the battery can be increased based
on the detected temperature rise gradient.

[0014]Also, in the aforementioned charging method, in the first charge
step, the battery can be charged at a constant current, and in the second
charge step, the battery can be charged at a constant voltage.

[0015]Also, in the aforementioned charging method, in the step in that a
voltage of the battery is detected, a charging current is controlled
based on the detected voltage, in that, if a voltage of the battery
increases to a predetermined voltage, a charging current for charging the
battery is reduced, and in that, after a voltage of the battery increases
to the predetermined voltage, a charging current is reduced, detection of
a voltage of the battery can be stopped in a predetermined period after
it is detected that a battery voltage increases to the predetermined
voltage.

[0016]Also, in the aforementioned charging method, a voltage of the
battery can be detected, and a charging current can be controlled based
on the detected voltage. In this case, if a voltage of the battery
increases to a predetermined voltage value, a charging current for
charging the battery can be reduced. Additionally, in this case, the
battery can be charged at smaller one of a charging current that is
controlled based on the detected voltage, and a charging current that is
controlled based on the prediction temperature.

[0017]The above and further objects of the present invention as well as
the features thereof will become more apparent from the following
detailed description to be made in conjunction with the accompanying
drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a graph showing an exemplary relationship of charging
current and maximum voltage to temperature of lithium-ion rechargeable
battery;

[0019]FIG. 2 is a graph showing charging current and battery temperature
of a battery that is charged in a charging method according to an
embodiment of the present invention;

[0020]FIG. 3 is a circuit diagram showing a battery pack that is charged
in the charging method according to the embodiment of the present
invention;

[0021]FIG. 4 is a flowchart showing the charging method according to the
embodiment of the present invention;

[0022]FIG. 5 is a graph showing the state where an arithmetic circuit
controls a charging current based on a battery voltage; and

[0023]FIG. 6 is a flowchart showing a charging method according to another
embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0024]FIG. 3 is a circuit diagram of a battery pack 10 that is charged in
a charging method according to the present invention. The battery pack 10
is attached to a charger unit 20, and is charged by a charging circuit 21
of a charger unit 20. The battery pack 10 includes an arithmetic circuit
12 that detects a voltage and a temperature of a battery 11, and controls
a current of the battery 11 based on the detected voltage and
temperature. The battery 11 is composed of a lithium-ion rechargeable
battery. The charging circuit 21 of the charger unit 20 includes a
constant-voltage and constant-current power supply 22, and a current
regulating circuit 23 that controls a charging current of the battery
that is provided from this constant-voltage and constant-current power
supply 22. The current regulating circuit 23 controls a charging current
of the battery to fully charge the battery 11 based on a control signal
that is provided from the arithmetic circuit 12 of the battery pack 10.

[0025]The arithmetic circuit 12 includes a voltage detecting circuit 13
that detects a voltage of the battery 11, a temperature detecting circuit
14 that detects a temperature of the battery 11, a remaining capacity
detecting circuit 15 that detects a remaining capacity of the battery 11,
and a memory 16 that stores predetermined voltages corresponding to
temperature ranges of the battery 11.

[0026]The voltage detecting circuit 13 detects a voltage between the both
ends of the battery 11 to detect a battery voltage. The temperature
detecting circuit 14 that detects a temperature of the battery 11 detects
a battery temperature based on electrical resistance of a temperature
sensor 17 that is thermally coupled to the battery 11. The remaining
capacity detecting circuit 15 detects a remaining capacity based on a
voltage of the battery 11 or an integrated current value and a voltage.
The memory 16 stores a low-temperature predetermined voltage
corresponding to a low temperature range, a normal predetermined voltage
corresponding to a normal temperature range, and a high-temperature
predetermined voltage corresponding to a high temperature range.

[0027]The arithmetic circuit 12 detects a temperature of the charged
battery 11 to detect a temperature gradient per unit time, and predicts a
battery temperature when the battery 11 will be charged to a first
predetermined capacity based on the temperature gradient. A temperature
of the battery at the first predetermined capacity is predicted as
follows. [0028]Battery Temperature: 15° C. [0029]Charging Current:
0.2 C [0030]Temperature Gradient of Charged Battery: 0.1° C./min.
[0031]Remaining Capacity of Battery in Current State: 1.0% [0032]First
Predetermined Capacity: 70% [0033]Rated Capacity of Battery: 600 mAh

[0034]If a battery has a remaining capacity of 10%, after additional 60%
charging operation, a remaining capacity of the battery will reach 70%.
Accordingly, a charging capacity that is necessary to charge this battery
to a remaining capacity of 70% is 60% of the rated capacity, i.e., 360
mAh. At 0.2 C, the charging capacity for 1 hour is 120 mAh. The charging
capacity for 1 minute is 120 mAh per 60 minutes. That is, at a charging
current of 0.2 C, the charging capacity of the battery for 1 minute is 2
mAh. Accordingly, at this charging capacity, it takes 180 minutes to
charge a capacity of 180 mAh. The temperature gradient is 0.1° C.
Accordingly, at this temperature gradient, after 180 minutes, the
temperature will rise 18° C. A temperature of the battery will
rise 18° C. from 15° C. Accordingly, it can be predicted
that a battery temperature will be 33° C. when the battery is
charged to 70% as the first predetermined capacity.

[0035]Also, the arithmetic circuit 12 detects a temperature of the charged
battery 11 to detect a temperature gradient per unit time, and predicts a
battery temperature when the battery 11 will be charged to a second
predetermined capacity based on the temperature gradient. The temperature
of the battery in the second predetermined capacity is predicted as
follows. [0036]Battery Temperature: 33° C. [0037]Charging Current:
0.2 C [0038]Temperature Gradient of Charged Battery: 0.1° C./min.
[0039]Remaining Capacity of Battery in Current State: 70% [0040]Second
Predetermined Capacity: 100% [0041]Rated Capacity of Battery: 600 mAh

[0042]If a battery has a remaining capacity of 70%, after additional 3.0%
charging operation, a remaining capacity of the battery will reach 100%.
Accordingly, a charging capacity that is necessary to charge this battery
to a remaining capacity of 100% is 30% of the rated capacity, i.e., 180
mAh. At 0.2 C, the charging capacity for 1 hour is 120 mAh. The charging
capacity for 1 minute is 120 mAh per 60 minutes. That is, at a charging
current of 0.2 C, the charging capacity of the battery for 1 minute is 2
mAh. Accordingly, at this charging capacity, it takes 90 minutes to
charge a capacity of 180 mAh. The temperature gradient is 0.1° C.
Accordingly, at this temperature gradient, after 90 minutes, a
temperature will rise 9° C. A temperature of the battery will rise
9° C. from 33° C. Accordingly, it can be predicted that a
battery temperature will be 42° C. when the battery is charged to
100% as the second predetermined capacity.

[0043]The arithmetic circuit 12 stores the temperature ranges to control a
charging current of the battery 11. For example, as shown in FIG. 2, the
arithmetic circuit 12 stores a first predetermined temperature T1
(45° C. in this Figure), a second predetermined temperature T2
(44° C. in this Figure), and a third predetermined temperature T3
(42° C. in this Figure). The arithmetic circuit 12 defines the
following first to fourth temperature ranges by dividing a temperature
zone by the first, second and third predetermined temperatures, and
controls a charging current according to these ranges. The first
temperature range is a range higher than the first predetermined
temperature T1. The second temperature range is a range between the first
predetermined temperature T1 and the second predetermined temperature T2.
The third temperature range is a range between the second predetermined
temperature T2 and the third predetermined temperature T3. The fourth
temperature range is a range lower than the third predetermined
temperature T3. If a prediction temperature of the charged battery falls
within the first temperature range, a charging current is set to the
minimum current. If a prediction temperature of the charged battery falls
within the second temperature range, a charging current is reduced a
predetermined amount. If a prediction temperature of the charged battery
falls within the third temperature range, the battery is charged without
changing a charging current. If a prediction temperature of the charged
battery falls within the fourth temperature range, a charging current is
increased a predetermined amount.

[0044]The aforementioned arithmetic circuit 12 controls a charging current
according to the divided four temperature ranges. However, the arithmetic
circuit does not necessarily control a charging current in the
aforementioned manner according to the divided four temperature ranges.
For example, the arithmetic circuit may have one predetermined
temperature. In this case, the arithmetic circuit reduces a current if a
temperature is higher than this predetermined temperature, and increases
a current if a temperature is lower than the predetermined temperature.
In addition to this, the arithmetic circuit may change a current value to
be reduced or increased to control a charging current as the temperature
difference between the predetermined temperature and a prediction
temperature increases.

[0045]In the aforementioned charging method, the battery is charged in a
manner shown in a flowchart of FIG. 4.

(Steps n=1 and 2)

[0046]A minimum current Imin is set to 0.2 C. Charging operation
starts in the state where a charging current I is 0.2 C as the minimum
current Imin.

(Steps n=3 and 4)

[0047]After 1 minute, a remaining capacity of the battery is detected.

(Steps n=5 and 6)

[0048]It is determined whether the battery is charged to a remaining
capacity of 70% as the first predetermined capacity. If the battery is
not charged to a remaining capacity of 70%, the procedure goes to Step
n=6, and a prediction temperature Ta when the battery will be charged to
70% is calculated based on a temperature rise gradient of the battery.

(Steps n=7 to 9)

[0049]If a remaining capacity of the battery is not less than 70%, the
procedure goes to Step n=7, and it is determined whether the battery is
charged to the fully-charged capacity as the second predetermined
capacity. If the battery is fully charged, the procedure goes to Step n=8
and the charging operation ends. If the battery is not fully charged, the
procedure goes to Step n=9, and a prediction temperature Ta when the
battery will be fully charged is calculated based on a temperature rise
gradient of the battery.

(Step n=10)

[0050]It is determined whether the prediction temperature Ta of the
battery falls within a range between the second predetermined temperature
T2 and the third predetermined temperature T3, in other words, whether
the prediction temperature Ta falls within the third temperature range.

[0051]If the prediction temperature Ta falls within the third temperature
range, the procedure returns to Step of n=3, and the charging operation
is continued without changing the charging current I.

(Steps n=11 and 12)

[0052]If the prediction temperature Ta does not fall within the third
temperature range, in Step n=11 it is determined whether the prediction
temperature Ta of the battery is not lower than the first predetermined
temperature T1, in other words, whether the prediction temperature Ta
falls within the first temperature range.

[0053]If the prediction temperature Ta falls within the first temperature
range, the procedure goes to Step of n=12, and the charging current I is
changed to 0.2 C as the minimum current Imin. After that, the procedure
returns to Step of n=3, and the charging operation is continued at the
changed charging current I.

(Steps n=13 to 15)

[0054]If the prediction temperature Ta does not fall within the first
temperature range, the procedure goes to Step of n=13, and it is
determined whether the prediction temperature Ta of the battery is not
lower than the second predetermined temperature T2.

[0055]If the prediction temperature Ta is not lower than the second
predetermined temperature T2, since it is already determined in Step of
n=11 that the prediction temperature Ta is lower than the first
predetermined temperature T1, it is determined that the prediction
temperature Ta falls within the second temperature range, and the
procedure goes to Step n=14 so that the charging current I is reduced 0.1
C. After that, the procedure returns to Step of n=3, and the charging
operation is continued at the changed charging current I.

[0056]If the prediction temperature Ta is lower than the second
predetermined temperature T2, since it is already determined in Step of
n=10 that the prediction temperature Ta is lower than the third
predetermined temperature T3, it is determined that the prediction
temperature Ta falls within the fourth temperature range, and the
procedure goes to Step n=15 so that the charging current I is increased
0.1 C. After that, the procedure returns to Step of n=3, and the charging
operation is continued at the changed charging current I.

[0057]In this embodiment, the charging operation is conducted in the state
where the battery is charged at a constant voltage and a constant
current. Under this charging operation, in the case where the charging
operation is conducted in a constant-voltage charging range, the battery
is charged in a manner as described in the aforementioned flow. However,
according to the charge characteristic of a battery itself, in the case
where the battery is required to be charged in the constant-voltage
charging range at a charging current lower than the current value
described in the aforementioned flow, the charging operation can be
conducted at this lower current value.

[0058]In this embodiment, in order to charge the battery at a current that
brings a battery temperature lower than the predetermined temperature, a
battery temperature at the first predetermined capacity (70%) is
predicted (first charging step), and in addition to this a battery
temperature at the second predetermined capacity (100%) is predicted
(second charging step). In the constant-voltage and constant-current
charging operation, the first predetermined capacity (70%) is a rough
value in that the operation shifts from constant current charging
operation to constant voltage charging operation. In the constant voltage
charging operation, a charging current gradually decreases. Accordingly,
a battery temperature is less likely to rise. However, a capacity in that
the operation shifts from the constant current charging operation to the
constant voltage charging operation is affected by temperature, battery
deterioration and the like, and is not fixed. For this reason, even in
the case where the battery is charged to the first predetermined capacity
(70%) or higher, the constant current charging operation may be
conducted. In this case, or due to other reason, a battery temperature
will rise. As shown in FIG. 1, if a battery temperature rises from the
normal temperature range to the high temperature range, charging voltage
and current in the high temperature range are lower than in the normal
temperature range. For this reason, there are problems in that a
completed charged amount of the battery is low, or charging operation
time is increased. In the case of a charging circuit that cannot reduce a
charging voltage, there is a problem in that the charging circuit stops
charging operation and as a result a completed charged amount of the
battery is low. Contrary to this, in this embodiment, in the case where
the battery is charged to the first predetermined capacity (70%) or
higher, even in the case where the constant current charging operation is
conducted, a battery temperature at the second predetermined capacity
(100%) is predicted, and a charging current is controlled based on the
predicted battery temperature (second charging step). Accordingly, the
aforementioned problems can be solved. Therefore, a completed charged
amount of the battery can be high, and charging operation time can be
short.

[0059]Additionally, the arithmetic circuit 12 detects a voltage of the
battery 11, and controls a charging current based on comparison of this
detection voltage with a predetermined value. FIG. 5 is a graph showing
the state where the arithmetic circuit 12 controls a charging current
based on a battery voltage. The arithmetic circuit 12 that controls a
current in a manner shown in this Figure reduces a charging current every
when a voltage of the battery 11 reaches a predetermined voltage (for
example, the predetermined voltage is 4.20 V, or can be 4.25 V or lower
in the normal temperature range of FIG. 1). When a cell voltage of the
battery 11 (the maximum battery voltage in the case where the battery is
composed of a plurality of serially-connected batteries) exceeds the
predetermined voltage, the arithmetic circuit 12 provides a current
reduction signal for reducing a current to the current regulating circuit
23 of the charger unit 20 through a communication terminal 18. The
current regulating circuit 23 of the charger unit 20 reduces a charging
current in response to this current reduction signal. However, the
current regulating circuits 23 have delay time to reduce a charging
current even when the battery pack 10 provides the current reduction
signal. In this delay time, a current of the battery 11 is not reduced,
and thus a voltage of the battery 11 increases. If a battery voltage
exceeds a maximum limit voltage, the battery charging operation stops.
Accordingly, if a battery voltage exceeds the maximum limit voltage
within this delay time, the charging operation will stop. Also, in order
to prevent this problem, the arithmetic circuit 12 has function that
turns a charging switch (not shown) OFF to temporarily stop charging
operation until the current regulating circuit 23 reduces a charging
current. Therefore, it is possible prevent that a battery voltage exceeds
the maximum limit voltage within the delay time. In order to prevent the
charging operation stop, the arithmetic circuit 12 can have a
cancellation circuit 19 that temporarily stops detecting a voltage of the
battery 11 in the delay time or stops providing the signal for stopping
charging operation in the delay time irrespective of voltage detection.

[0060]A charging current of the battery is controlled based on both a
prediction temperature and a battery voltage. A charging current is
controlled and adjusted to a lower current value of current-values that
are determined based on the prediction temperature and the battery
voltage.

[0061]In this charging method, the battery is charged in a manner shown in
a flowchart of FIG. 6.

(Step n=1)

[0062]Charging operation starts.

(Step n=2)

[0063]The arithmetic circuit 12 determines a charging current IT
based on a prediction battery temperature Ta. As discussed above, a
temperature rise gradient of the battery is detected at a current that
charges the battery, and a temperature of the battery is predicted based
on this temperature rise gradient when where the battery will be charged
to a predetermined capacity. A charging current IT is determined so
that a prediction temperature Ta may be lower than a predetermined
temperature.

(Step n=3)

[0064]The arithmetic circuit 12 determines a charging current IV
based on a battery voltage. The charging current IV is determined to
be reduced when a voltage of the battery increases to the predetermined
voltage so that a battery voltage may not exceed the predetermined
voltage.

(Steps n=4 to 6)

[0065]The charging current IV that is determined based on a battery
voltage is compared with the charging current IT that is determined
based on a prediction temperature Ta. The battery is charged at a lower
current value of the charging current IT and the charging current
IV as a charging current I.

(Step n=7)

[0066]It is determined whether the battery is fully charged. If the
battery is not fully charged, the procedure returns to Step n=2, and the
charging operation is continued. If the battery is fully charged, the
procedure goes to Step n=8, and the charging operation ends.

[0067]It should be apparent to those with an ordinary skill in the art
that while various preferred embodiments of the invention have been shown
and described, it is contemplated that the invention is not limited to
the particular embodiments disclosed, which are deemed to be merely
illustrative of the inventive concepts and should not be interpreted as
limiting the scope of the invention, and which are suitable for all
modifications and changes falling within the scope of the invention as
defined in the appended claims. The present application is based on
Application No. 2007-321,374 filed in Japan on Dec. 12, 2007, the content
of which is incorporated herein by reference.